1. Field of the Invention
The present invention is directed to device and method for storing ink in a reticulated foam for use in ink-jet printing, and is particularly directed to reticulated foam that has a volume, before being inserted in the ink chamber, that is close to its net volume after insertion.
2. Description of the Related Art
Ink-jet printers commonly employ ink-jet print cartridges, or "pens," which include a sophisticated printhead and an attached ink chamber filled with a supply of ink. The printhead is a micromechanical part that contains an array of miniature thermal resistors or piezoelectric transducers that are energized to eject small droplets of ink out of an associated array of nozzles. In some cases the printhead is permanently attached to the ink supply, and in others the ink supply can be separately replaced. The pen is mounted in a carriage in the printer where the pen electrically interfaces with the printer. The printer scans the pen back and forth across the print medium (e.g., paper) as the pen ejects small droplets from the nozzles in selected matrix patterns, to thereby print a swath of the desired alphanumeric characters or graphics. After each swath of printing, the printer advances the medium incrementally to begin a new swath. Successive swaths are printed in this manner to complete the desired alphanumeric characters or graphics on the medium.
The ink in the pen must be held in the ink chamber at less than atmospheric pressure, so that the ink does not drool out of the nozzles when the nozzles are not firing. However, this negative relative pressure, or backpressure, must not be so great that air is gulped into the interior of the firing chambers, thereby causing them to "deprime" and no longer function. Various mechanisms have been devised to provide the appropriate backpressure, such as resilient bladders and combinations of springs and flexible bags.
One of the most reliable backpressure systems uses a porous material, such as synthetic foam, in the ink tank. Ink is injected into the foam and the foam retains the ink at the appropriate backpressure by capillary action. U.S. Pat. No. 4,771,295 (Baker '295), which is assigned to Hewlett-Packard Company (the assignee of the present invention), discloses an ink-jet pen that uses synthetic foam for ink retention and backpressure. A key feature of the pen disclosed in Baker '295 is an ink pipe that extends upward from a bottom wall of the pen body and into compressive contact with the foam. The ink pipe is the fluid conduit for the ink from the foam to the printhead. The ink pipe locally compresses the foam to thereby increase its capillarity in the region of the ink pipe. As ink is depleted from the foam, the increased capillarity near the ink pipe tends to draw ink from all other portions of the foam toward the ink pipe, so that the maximum amount of ink can be drawn from the foam for printing.
It is important in foam-based pens to keep the foam in secure contact with the ink pipe to maintain the compressive capillarity and the ink seal between the pipe and the ink-filled foam. If this compressive contact is broken an air path forms from the ambient air around the sides of the foam and into the interior of the ink pipe, resulting in a catastrophic deprime of the pen. To ensure adequate compressive contact between the ink pipe and the foam, prior art pens have employed bodies of foam that are larger in volume than the ink chamber they are to fill. The foam body must therefore be compressed by some mechanism during insertion. Once inserted, the compression is released so that the sides of the foam can expand into resilient contact with the walls of the ink chamber. This compressive contact with the walls of the ink chamber keeps the foam in place and in contact with the ink pipe. In many cases, such foam bodies have pre-insertion volumes that can be 50% or more greater than the interior volume of the ink chamber.
It is very important to minimize or avoid wrinkling the foam or causing other compression anomalies in the foam during the process of inserting the foam into the ink chamber. Such localized wrinkles and anomalies introduce unintended regions of higher compression. These regions of increased compression will have a higher capillarity than the surrounding foam, and ink will be stranded at these locations. Ink thus stranded is wasted and will not be available for printing.
The mechanisms needed to insert foam members that have large volumes relative to the internal volume of the ink chamber add complexity and expense to the assembly of the pen. One such mechanism, for example, involves parallel plates that squeeze the foam to a width smaller than the ink chamber. After the plates and the foam are lowered into the ink chamber, a separate mechanism pushes the back of the foam while the plates are removed from the pen body in a shuffling or ratcheting motion.
The complexity of foam insertion mechanisms is compounded as ink chambers become taller and more narrow. It has become increasingly important to make ink-jet pens as narrow as possible. The overall width of the pen influences the width of the printer and the amount of desk space the printer takes up. In addition, when printing with multiple pens, such as in color printing, print quality can be enhanced by making the pens narrower. Narrower pens allow the printheads of the pens to be more closely spaced so that during printing, as the pen is scanned across the print medium, less time goes by between ejection of the different colored droplets. On the other hand, users of printers desire that ink-jet pens last longer, in other words, that they hold more ink. Often the best way of increasing the volume of ink while maintaining a narrow profile is to make the pen taller.
However, the taller and more narrow the pen is, the more difficult it is to properly insert foam into the ink chamber. Particularly with pens that load the foam from the top down into the pen, the taller the pen is, the more likely it will be that undesirable wrinkles or other anomalies will form in the foam as the foam is loaded into the pen. Also, the taller the pen is, the more chance there is that friction between the foam and the interior walls of the pen body during insertion will impede the desired compressive force between the foam and the ink pipe. The mechanisms needed to insert foam bodies that have a substantially greater pre-insertion volume than the ink chamber where the ink chamber is tall and narrow may be prohibitively complex.
There remains a need for a foam-based ink containment device that avoids the problems associated with insertion of large foam bodies, and which at the same time provides adequate sealing between the foam and the ink pipe. This solution would preferably allow for top-down insertion into ink-jet pens having tall, narrow aspect ratios.